1. Field of the Invention
This invention relates to brazed aluminum, and particularly to the corrosion properties of vacuum-brazed aluminum materials.
2. Description of the Prior Art
Aluminum has long been known for its resistance to corrosion, a property which renders it a favorable material of construction for a wide variety of purposes. With the relatively recent development of vacuum brazing technology for joining together aluminum parts, aluminum has gained wide acceptance in the manufacture of heat exchangers such as automotive radiators and evaporators for air conditioning units. Vacuum brazing has considerable advantages over the earlier-developed technique of flux brazing, since it avoids the need for a special brazing flux and for expensive cleaning operations to remove the flux after brazing, and it eliminates the possibility of flux contamination corroding the aluminum.
A problem which arises in all brazing operations is the intergranular penetration of core material by certain species in the brazing alloy, notably silicon. When the brazed product is subjected to a corrosive environment, the intergranular regions in the core, where silicon is present in high concentrations, are particularly susceptible to corrosion. In automotive radiators, for example, where the brazing alloy is present only on the exterior surface of the tubes, the salts and moisture from the road are sufficiently corrosive to cause attack from the outside, resulting ultimately in pinhole formation.
This problem has been addressed in the literature in a variety of ways. An early example is Miller, U.S. Pat. No. 2,821,014 (Jan. 28, 1958), where it is disclosed that intergranular corrosion problems in flux and dip brazing are alleviated by adding an interlayer between the structural member portion and the brazing layer. The interlayer is aluminum or an aluminum-base alloy, particularly certain magnesium-containing alloys, having a melting point greater than that of the structural alloy. The solution offered by Singleton et al., U.S. Pat. No. 3,788,824 (Jan. 29, 1974) and its divisional, U.S. Pat. No. 3,881,879 (May 6, 1975), is directed to vacuum brazing, and involves the addition of iron to either the core alloy or the cladding alloy as an alloying element, resulting in improvements in both corrosion resistance and sag resistance. Anthony et al., U.S. Pat. No. 4,039,298 (Aug. 2, 1977) address both flux and vacuum brazing, and disclose a composite of complex and highly specified composition as being particularly beneficial in terms of corrosion properties. The disclosed core alloy contains specified amounts of manganese, copper, chromium, silicon and iron as alloying elements with both a solid solution and an alpha-phase, whereas the alloying elements in the cladding are bismuth and silicon. An additional disclosure by the same patentees appears in U.S. Pat. No. 4,093,782 (June 6, 1978) and its continuation-in-part, U.S. Pat. No. 4,167,410 (Sept. 11, 1979), in which the core alloy contains a specified combination of chromium and manganese, with resultant improvements in both corrosion resistance and sag resistance. A similar disclosure appears in Setzer et al., U.S. Pat. No. 3,994,695 (Nov. 30, 1976), where the core alloy contains a chromium-manganese-zirconium combination, the sole claimed benefit however being an improvement in sag resistance. A combination of copper and titanium as primary alloying elements in the core alloy is disclosed in Kaifu et al., U.S. Pat. No. 4,339,510 (July 13, 1982), as providing a benefit in intergranular corrosion resistance.
A different approach is disclosed by Nakamura, U.S. Pat. No. 4,172,548 (Oct. 30, 1979), in which corrosion following fluxless brazing processes in general (including both vacuum brazing and brazing in an inert atmosphere) is controlled by controlling the grain size of the brazing sheet to at least 60 microns in diameter, achieved by a controlled cold work followed by a full anneal.
To summarize, with the exception of Nakamura, prior investigators have approached the problem by introducing specific elements in the alloy composition, rather than introducing processing modifications. Processing modifications, on the other hand, particularly variations in the combinations, degrees and sequence of strain hardening and annealing, are generally used for controlling the ductility and tensile properties of the final product. Setzer et al., referenced above, demonstrates several of these combinations, ranging from those ending with a fully hardened product (maximum cold work) to those ending with a fully strain-free (annealed) product.
In manufacturing operations which require forming of the sheet into various shapes prior to brazing, a certain degree of ductility is required for the sheet to respond in an appropriate manner to the forming equipment. Examples of such forming processes are roll forming, drawing, spinning and shear forming. Conventional methods of achieving the desired degree of ductility involve a partial anneal of the product after it has been cold rolled down to the final gauge. An explanation and description of this is disclosed by Singleton, U.S. Pat. No. 3,963,454 (June 15, 1976) at column 4, lines 34-58. The partial anneal is controlled to leave the desired amount of cold work remaining in the product.